Wind energy is increasingly recognized as a viable option for complementing and even replacing other types of energy such as fossil fuels. In the early development of wind energy, the majority of wind turbines were constructed for operation at a constant speed. However, recently the trend is toward using variable-speed wind turbines to better capture available wind power. In most cases, wind turbine blade pitch angles can be adjusted to control the operation of the variable speed wind turbine.
Wind turbine manufacturers use variable-speed turbines to capture available wind power over a wide range of wind speeds. To be effective, though, these variable speed wind turbines require active control systems to react to changing wind and other operating conditions. One concept that is fundamental to the control dynamics for a wind generator is that changing speed is a relatively slow process due to the large inertia values involved, and this makes it difficult to use a power converter in the wind turbine or in the electrical power plant of the wind turbine to control the propeller speed. As a result, manufacturers and operators of variable speed wind turbines also use a blade pitch control on an ongoing basis to regulate the power flow at the high speed limit. In other words, a control system is used to vary pitch rapidly in response to propeller speed, and significant efforts have been made to improve this ongoing pitch control system.
The power coefficient Cp for a wind turbine is a measure of the mechanical power delivered by the propeller to a low sped shaft of a wind turbine. Cp is defined as the ratio of the received mechanical power to the power available in the wind.
WO2007123552 discloses an adaptive control method for operating a wind turbine in variable speed conditions in order to adjust the blade pitch angle for better capturing the available wind power. The method includes determining captured power for a period of time for the operating wind turbine. For this period of time, the wind power available is also determined by means of a wind sensor being mounted for instance at the mast of the wind turbine. Then, the power coefficient Cp is determined based on the ratio of captured power to the available wind power. The described adaptive control method continues with generating a blade pitch angle signal for use in setting a blade pitch angle of one or more blades in the wind turbine by modifying an existing blade pitch angle setting by a pitch increment. Then, the captured power Cp and the available wind power are determined for a next or second time period. The power coefficient Cp is determined for this second time period and is compared to the earlier power coefficient. Based on this comparison, the next blade pitch angle signal is generated to set the blades to a new pitch angle that differs from the existing pitch angle by a next or second pitch increment. Specifically, if the power coefficient is increasing (i.e., the second power coefficient is larger than the first power coefficient), a sign of the prior pitch angle increment is retained and a next pitch angle increment is added to the existing blade pitch angle. If the power coefficient is decreasing, then the sign of the pitch angle increment is changed prior to it being added to the existing blade pitch angle for the blades. The process continues with a next power coefficient Cp being determined after another adaptation time period has lapsed and further modifying the blade pitch angle by a pitch angle increment. The known described blade pitch angle adjustment method has the disadvantage that it relies on two completely different measurement processes, i.e. the measurement process for the actual captured power and the measurement of the in principle available wind power. If only one of these measurement processes defective, whole blade pitch angle adjustment method leads to unreasonable results.